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1=head1 NAME
2
3perlvar - Perl predefined variables
4
5=head1 DESCRIPTION
6
7=head2 Predefined Names
8
5a964f20 9The following names have special meaning to Perl. Most
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10punctuation names have reasonable mnemonics, or analogs in the
11shells. Nevertheless, if you wish to use long variable names,
12you need only say
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13
14 use English;
15
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16at the top of your program. This aliases all the short names to the long
17names in the current package. Some even have medium names, generally
18borrowed from B<awk>. In general, it's best to use the
a0d0e21e 19
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20 use English '-no_match_vars';
21
22invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids
23a certain performance hit with the use of regular expressions. See
24L<English>.
25
26Variables that depend on the currently selected filehandle may be set by
27calling an appropriate object method on the IO::Handle object, although
28this is less efficient than using the regular built-in variables. (Summary
29lines below for this contain the word HANDLE.) First you must say
a0d0e21e 30
19799a22 31 use IO::Handle;
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32
33after which you may use either
34
35 method HANDLE EXPR
36
5a964f20 37or more safely,
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38
39 HANDLE->method(EXPR)
40
14218588 41Each method returns the old value of the IO::Handle attribute.
a1ce9542 42The methods each take an optional EXPR, which, if supplied, specifies the
19799a22 43new value for the IO::Handle attribute in question. If not supplied,
14218588 44most methods do nothing to the current value--except for
a0d0e21e 45autoflush(), which will assume a 1 for you, just to be different.
a1ce9542 46
14218588 47Because loading in the IO::Handle class is an expensive operation, you should
19799a22 48learn how to use the regular built-in variables.
a0d0e21e 49
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50A few of these variables are considered "read-only". This means that if
51you try to assign to this variable, either directly or indirectly through
52a reference, you'll raise a run-time exception.
a0d0e21e 53
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54You should be very careful when modifying the default values of most
55special variables described in this document. In most cases you want
56to localize these variables before changing them, since if you don't,
57the change may affect other modules which rely on the default values
58of the special variables that you have changed. This is one of the
59correct ways to read the whole file at once:
60
61 open my $fh, "foo" or die $!;
62 local $/; # enable localized slurp mode
63 my $content = <$fh>;
64 close $fh;
65
66But the following code is quite bad:
67
68 open my $fh, "foo" or die $!;
69 undef $/; # enable slurp mode
70 my $content = <$fh>;
71 close $fh;
72
73since some other module, may want to read data from some file in the
74default "line mode", so if the code we have just presented has been
75executed, the global value of C<$/> is now changed for any other code
76running inside the same Perl interpreter.
77
78Usually when a variable is localized you want to make sure that this
79change affects the shortest scope possible. So unless you are already
80inside some short C<{}> block, you should create one yourself. For
81example:
82
83 my $content = '';
84 open my $fh, "foo" or die $!;
85 {
86 local $/;
87 $content = <$fh>;
88 }
89 close $fh;
90
91Here is an example of how your own code can go broken:
92
93 for (1..5){
94 nasty_break();
95 print "$_ ";
96 }
97 sub nasty_break {
98 $_ = 5;
99 # do something with $_
100 }
101
102You probably expect this code to print:
103
104 1 2 3 4 5
105
106but instead you get:
107
108 5 5 5 5 5
109
110Why? Because nasty_break() modifies C<$_> without localizing it
111first. The fix is to add local():
112
113 local $_ = 5;
114
115It's easy to notice the problem in such a short example, but in more
116complicated code you are looking for trouble if you don't localize
117changes to the special variables.
118
fb73857a 119The following list is ordered by scalar variables first, then the
87275199 120arrays, then the hashes.
fb73857a 121
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122=over 8
123
124=item $ARG
125
126=item $_
a054c801 127X<$_> X<$ARG>
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128
129The default input and pattern-searching space. The following pairs are
130equivalent:
131
19799a22 132 while (<>) {...} # equivalent only in while!
54310121 133 while (defined($_ = <>)) {...}
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134
135 /^Subject:/
136 $_ =~ /^Subject:/
137
138 tr/a-z/A-Z/
139 $_ =~ tr/a-z/A-Z/
140
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141 chomp
142 chomp($_)
a0d0e21e 143
54310121 144Here are the places where Perl will assume $_ even if you
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145don't use it:
146
147=over 3
148
149=item *
150
151Various unary functions, including functions like ord() and int(), as well
152as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
153STDIN.
154
155=item *
156
157Various list functions like print() and unlink().
158
159=item *
160
161The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
162without an C<=~> operator.
163
54310121 164=item *
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165
166The default iterator variable in a C<foreach> loop if no other
167variable is supplied.
168
54310121 169=item *
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170
171The implicit iterator variable in the grep() and map() functions.
172
54310121 173=item *
cb1a09d0 174
c47ff5f1 175The default place to put an input record when a C<< <FH> >>
cb1a09d0 176operation's result is tested by itself as the sole criterion of a C<while>
14218588 177test. Outside a C<while> test, this will not happen.
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178
179=back
180
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181As C<$_> is a global variable, this may lead in some cases to unwanted
182side-effects. As of perl 5.9.1, you can now use a lexical version of
183C<$_> by declaring it in a file or in a block with C<my>. Moreover,
184declaring C<our $> restores the global C<$_> in the current scope.
185
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186(Mnemonic: underline is understood in certain operations.)
187
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188=back
189
190=over 8
191
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192=item $a
193
194=item $b
a054c801 195X<$a> X<$b>
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196
197Special package variables when using sort(), see L<perlfunc/sort>.
198Because of this specialness $a and $b don't need to be declared
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199(using use vars, or our()) even when using the C<strict 'vars'> pragma.
200Don't lexicalize them with C<my $a> or C<my $b> if you want to be
201able to use them in the sort() comparison block or function.
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202
203=back
204
205=over 8
206
c47ff5f1 207=item $<I<digits>>
a054c801 208X<$1> X<$2> X<$3>
a0d0e21e 209
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210Contains the subpattern from the corresponding set of capturing
211parentheses from the last pattern match, not counting patterns
212matched in nested blocks that have been exited already. (Mnemonic:
213like \digits.) These variables are all read-only and dynamically
214scoped to the current BLOCK.
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215
216=item $MATCH
217
218=item $&
a054c801 219X<$&> X<$MATCH>
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220
221The string matched by the last successful pattern match (not counting
222any matches hidden within a BLOCK or eval() enclosed by the current
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223BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
224and dynamically scoped to the current BLOCK.
a0d0e21e 225
19ddd453 226The use of this variable anywhere in a program imposes a considerable
667e1aea 227performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 228
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229See L</@-> for a replacement.
230
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231=item $PREMATCH
232
233=item $`
a054c801 234X<$`> X<$PREMATCH>
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235
236The string preceding whatever was matched by the last successful
237pattern match (not counting any matches hidden within a BLOCK or eval
a8f8344d 238enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
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239string.) This variable is read-only.
240
19ddd453 241The use of this variable anywhere in a program imposes a considerable
667e1aea 242performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 243
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244See L</@-> for a replacement.
245
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246=item $POSTMATCH
247
248=item $'
a054c801 249X<$'> X<$POSTMATCH>
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250
251The string following whatever was matched by the last successful
252pattern match (not counting any matches hidden within a BLOCK or eval()
a8f8344d 253enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
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254string.) Example:
255
22d0716c 256 local $_ = 'abcdefghi';
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257 /def/;
258 print "$`:$&:$'\n"; # prints abc:def:ghi
259
19799a22 260This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 261
19ddd453 262The use of this variable anywhere in a program imposes a considerable
667e1aea 263performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 264
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265See L</@-> for a replacement.
266
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267=item $LAST_PAREN_MATCH
268
269=item $+
a054c801 270X<$+> X<$LAST_PAREN_MATCH>
a0d0e21e 271
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272The text matched by the last bracket of the last successful search pattern.
273This is useful if you don't know which one of a set of alternative patterns
274matched. For example:
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275
276 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
277
278(Mnemonic: be positive and forward looking.)
19799a22 279This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 280
a01268b5 281=item $^N
a054c801 282X<$^N>
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283
284The text matched by the used group most-recently closed (i.e. the group
285with the rightmost closing parenthesis) of the last successful search
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286pattern. (Mnemonic: the (possibly) Nested parenthesis that most
287recently closed.)
288
210b36aa 289This is primarily used inside C<(?{...})> blocks for examining text
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290recently matched. For example, to effectively capture text to a variable
291(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
292
293 (?:(...)(?{ $var = $^N }))
294
295By setting and then using C<$var> in this way relieves you from having to
296worry about exactly which numbered set of parentheses they are.
297
298This variable is dynamically scoped to the current BLOCK.
299
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300=item @LAST_MATCH_END
301
6cef1e77 302=item @+
a054c801 303X<@+> X<@LAST_MATCH_END>
6cef1e77 304
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305This array holds the offsets of the ends of the last successful
306submatches in the currently active dynamic scope. C<$+[0]> is
307the offset into the string of the end of the entire match. This
308is the same value as what the C<pos> function returns when called
309on the variable that was matched against. The I<n>th element
310of this array holds the offset of the I<n>th submatch, so
311C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
312past where $2 ends, and so on. You can use C<$#+> to determine
313how many subgroups were in the last successful match. See the
314examples given for the C<@-> variable.
6cef1e77 315
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316=item %+
317X<%+>
318
319Similar to C<@+>, the C<%+> hash allows access to the named capture
320buffers, should they exist, in the last successful match in the
321currently active dynamic scope.
322
323C<$+{foo}> is equivalent to C<$1> after the following match:
324
325 'foo'=~/(?<foo>foo)/;
326
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327The underlying behaviour of %+ is provided by the L<re::Tie::Hash::NamedCapture>
328module.
329
330B<Note:> As C<%-> and C<%+> are tied views into a common internal hash
331associated with the last successful regular expression. Therefore mixing
332iterative access to them via C<each> may have unpredictable results.
333Likewise, if the last successful match changes then the results may be
334surprising.
335
fcc7d916 336=item HANDLE->input_line_number(EXPR)
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337
338=item $INPUT_LINE_NUMBER
339
340=item $NR
341
342=item $.
a054c801 343X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
a0d0e21e 344
81714fb9 345Current line number for the last filehandle accessed.
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346
347Each filehandle in Perl counts the number of lines that have been read
348from it. (Depending on the value of C<$/>, Perl's idea of what
349constitutes a line may not match yours.) When a line is read from a
350filehandle (via readline() or C<< <> >>), or when tell() or seek() is
351called on it, C<$.> becomes an alias to the line counter for that
352filehandle.
353
354You can adjust the counter by assigning to C<$.>, but this will not
355actually move the seek pointer. I<Localizing C<$.> will not localize
356the filehandle's line count>. Instead, it will localize perl's notion
357of which filehandle C<$.> is currently aliased to.
358
359C<$.> is reset when the filehandle is closed, but B<not> when an open
360filehandle is reopened without an intervening close(). For more
e48df184 361details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
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362an explicit close, line numbers increase across ARGV files (but see
363examples in L<perlfunc/eof>).
364
365You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
366line counter for a given filehandle without having to worry about
367which handle you last accessed.
368
369(Mnemonic: many programs use "." to mean the current line number.)
370
371=item IO::Handle->input_record_separator(EXPR)
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372
373=item $INPUT_RECORD_SEPARATOR
374
375=item $RS
376
377=item $/
a054c801 378X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
a0d0e21e 379
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380The input record separator, newline by default. This
381influences Perl's idea of what a "line" is. Works like B<awk>'s RS
19799a22 382variable, including treating empty lines as a terminator if set to
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383the null string. (An empty line cannot contain any spaces
384or tabs.) You may set it to a multi-character string to match a
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385multi-character terminator, or to C<undef> to read through the end
386of file. Setting it to C<"\n\n"> means something slightly
387different than setting to C<"">, if the file contains consecutive
388empty lines. Setting to C<""> will treat two or more consecutive
389empty lines as a single empty line. Setting to C<"\n\n"> will
390blindly assume that the next input character belongs to the next
14218588 391paragraph, even if it's a newline. (Mnemonic: / delimits
19799a22 392line boundaries when quoting poetry.)
a0d0e21e 393
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394 local $/; # enable "slurp" mode
395 local $_ = <FH>; # whole file now here
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396 s/\n[ \t]+/ /g;
397
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398Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
399better for something. :-)
68dc0745 400
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401Setting C<$/> to a reference to an integer, scalar containing an integer, or
402scalar that's convertible to an integer will attempt to read records
5b2b9c68 403instead of lines, with the maximum record size being the referenced
19799a22 404integer. So this:
5b2b9c68 405
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406 local $/ = \32768; # or \"32768", or \$var_containing_32768
407 open my $fh, $myfile or die $!;
408 local $_ = <$fh>;
5b2b9c68 409
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410will read a record of no more than 32768 bytes from FILE. If you're
411not reading from a record-oriented file (or your OS doesn't have
412record-oriented files), then you'll likely get a full chunk of data
413with every read. If a record is larger than the record size you've
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414set, you'll get the record back in pieces. Trying to set the record
415size to zero or less will cause reading in the (rest of the) whole file.
5b2b9c68 416
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417On VMS, record reads are done with the equivalent of C<sysread>,
418so it's best not to mix record and non-record reads on the same
419file. (This is unlikely to be a problem, because any file you'd
83763826 420want to read in record mode is probably unusable in line mode.)
14218588 421Non-VMS systems do normal I/O, so it's safe to mix record and
19799a22 422non-record reads of a file.
5b2b9c68 423
14218588 424See also L<perlport/"Newlines">. Also see C<$.>.
883faa13 425
fcc7d916 426=item HANDLE->autoflush(EXPR)
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427
428=item $OUTPUT_AUTOFLUSH
429
430=item $|
a054c801 431X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
a0d0e21e 432
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433If set to nonzero, forces a flush right away and after every write
434or print on the currently selected output channel. Default is 0
14218588 435(regardless of whether the channel is really buffered by the
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436system or not; C<$|> tells you only whether you've asked Perl
437explicitly to flush after each write). STDOUT will
438typically be line buffered if output is to the terminal and block
439buffered otherwise. Setting this variable is useful primarily when
440you are outputting to a pipe or socket, such as when you are running
441a Perl program under B<rsh> and want to see the output as it's
442happening. This has no effect on input buffering. See L<perlfunc/getc>
443for that. (Mnemonic: when you want your pipes to be piping hot.)
a0d0e21e 444
46550894 445=item IO::Handle->output_field_separator EXPR
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446
447=item $OUTPUT_FIELD_SEPARATOR
448
449=item $OFS
450
451=item $,
a054c801 452X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
a0d0e21e 453
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454The output field separator for the print operator. If defined, this
455value is printed between each of print's arguments. Default is C<undef>.
456(Mnemonic: what is printed when there is a "," in your print statement.)
a0d0e21e 457
46550894 458=item IO::Handle->output_record_separator EXPR
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459
460=item $OUTPUT_RECORD_SEPARATOR
461
462=item $ORS
463
464=item $\
a054c801 465X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
a0d0e21e 466
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467The output record separator for the print operator. If defined, this
468value is printed after the last of print's arguments. Default is C<undef>.
469(Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
470Also, it's just like C<$/>, but it's what you get "back" from Perl.)
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471
472=item $LIST_SEPARATOR
473
474=item $"
a054c801 475X<$"> X<$LIST_SEPARATOR>
a0d0e21e 476
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477This is like C<$,> except that it applies to array and slice values
478interpolated into a double-quoted string (or similar interpreted
479string). Default is a space. (Mnemonic: obvious, I think.)
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480
481=item $SUBSCRIPT_SEPARATOR
482
483=item $SUBSEP
484
485=item $;
a054c801 486X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
a0d0e21e 487
54310121 488The subscript separator for multidimensional array emulation. If you
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489refer to a hash element as
490
491 $foo{$a,$b,$c}
492
493it really means
494
495 $foo{join($;, $a, $b, $c)}
496
497But don't put
498
499 @foo{$a,$b,$c} # a slice--note the @
500
501which means
502
503 ($foo{$a},$foo{$b},$foo{$c})
504
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505Default is "\034", the same as SUBSEP in B<awk>. If your
506keys contain binary data there might not be any safe value for C<$;>.
a0d0e21e 507(Mnemonic: comma (the syntactic subscript separator) is a
19799a22 508semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
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509taken for something more important.)
510
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511Consider using "real" multidimensional arrays as described
512in L<perllol>.
a0d0e21e 513
fcc7d916 514=item HANDLE->format_page_number(EXPR)
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515
516=item $FORMAT_PAGE_NUMBER
517
518=item $%
a054c801 519X<$%> X<$FORMAT_PAGE_NUMBER>
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520
521The current page number of the currently selected output channel.
19799a22 522Used with formats.
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523(Mnemonic: % is page number in B<nroff>.)
524
fcc7d916 525=item HANDLE->format_lines_per_page(EXPR)
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526
527=item $FORMAT_LINES_PER_PAGE
528
529=item $=
a054c801 530X<$=> X<$FORMAT_LINES_PER_PAGE>
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531
532The current page length (printable lines) of the currently selected
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533output channel. Default is 60.
534Used with formats.
535(Mnemonic: = has horizontal lines.)
a0d0e21e 536
fcc7d916 537=item HANDLE->format_lines_left(EXPR)
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538
539=item $FORMAT_LINES_LEFT
540
541=item $-
a054c801 542X<$-> X<$FORMAT_LINES_LEFT>
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543
544The number of lines left on the page of the currently selected output
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545channel.
546Used with formats.
547(Mnemonic: lines_on_page - lines_printed.)
a0d0e21e 548
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549=item @LAST_MATCH_START
550
6cef1e77 551=item @-
a054c801 552X<@-> X<@LAST_MATCH_START>
6cef1e77 553
19799a22 554$-[0] is the offset of the start of the last successful match.
6cef1e77 555C<$-[>I<n>C<]> is the offset of the start of the substring matched by
8f580fb8 556I<n>-th subpattern, or undef if the subpattern did not match.
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557
558Thus after a match against $_, $& coincides with C<substr $_, $-[0],
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559$+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
560$+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
561C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
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562matched subgroup in the last successful match. Contrast with
563C<$#+>, the number of subgroups in the regular expression. Compare
19799a22 564with C<@+>.
6cef1e77 565
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566This array holds the offsets of the beginnings of the last
567successful submatches in the currently active dynamic scope.
568C<$-[0]> is the offset into the string of the beginning of the
569entire match. The I<n>th element of this array holds the offset
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570of the I<n>th submatch, so C<$-[1]> is the offset where $1
571begins, C<$-[2]> the offset where $2 begins, and so on.
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572
573After a match against some variable $var:
574
575=over 5
576
4375e838 577=item C<$`> is the same as C<substr($var, 0, $-[0])>
4ba05bdc 578
4375e838 579=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
4ba05bdc 580
4375e838 581=item C<$'> is the same as C<substr($var, $+[0])>
4ba05bdc
GS
582
583=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
584
585=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
586
80dc6883 587=item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
4ba05bdc
GS
588
589=back
590
44a2ac75
YO
591=item %-
592X<%->
593
594Similar to %+, this variable allows access to the named capture
595buffers that were defined in the last successful match. It returns
596a reference to an array containing one value per buffer of a given
597name in the pattern.
598
599 if ('1234'=~/(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
600 foreach my $name (sort keys(%-)) {
601 my $ary = $-{$name};
602 foreach my $idx (0..$#$ary) {
603 print "\$-{$name}[$idx] : ",
604 (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"),
605 "\n";
606 }
607 }
608 }
609
610would print out:
611
612 $-{A}[0] : '1'
613 $-{A}[1] : '3'
614 $-{B}[0] : '2'
615 $-{B}[1] : '4'
616
617The behaviour of %- is implemented via the L<re::Tie::Hash::NamedCapture> module.
618
619Note that C<%-> and C<%+> are tied views into a common internal hash
620associated with the last successful regular expression. Therefore mixing
621iterative access to them via C<each> may have unpredictable results.
622Likewise, if the last successful match changes then the results may be
623surprising.
624
fcc7d916 625=item HANDLE->format_name(EXPR)
a0d0e21e
LW
626
627=item $FORMAT_NAME
628
629=item $~
a054c801 630X<$~> X<$FORMAT_NAME>
a0d0e21e
LW
631
632The name of the current report format for the currently selected output
14218588 633channel. Default is the name of the filehandle. (Mnemonic: brother to
19799a22 634C<$^>.)
a0d0e21e 635
fcc7d916 636=item HANDLE->format_top_name(EXPR)
a0d0e21e
LW
637
638=item $FORMAT_TOP_NAME
639
640=item $^
a054c801 641X<$^> X<$FORMAT_TOP_NAME>
a0d0e21e
LW
642
643The name of the current top-of-page format for the currently selected
14218588 644output channel. Default is the name of the filehandle with _TOP
a0d0e21e
LW
645appended. (Mnemonic: points to top of page.)
646
46550894 647=item IO::Handle->format_line_break_characters EXPR
a0d0e21e
LW
648
649=item $FORMAT_LINE_BREAK_CHARACTERS
650
651=item $:
a054c801 652X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
a0d0e21e
LW
653
654The current set of characters after which a string may be broken to
54310121 655fill continuation fields (starting with ^) in a format. Default is
a0d0e21e
LW
656S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
657poetry is a part of a line.)
658
46550894 659=item IO::Handle->format_formfeed EXPR
a0d0e21e
LW
660
661=item $FORMAT_FORMFEED
662
663=item $^L
a054c801 664X<$^L> X<$FORMAT_FORMFEED>
a0d0e21e 665
14218588 666What formats output as a form feed. Default is \f.
a0d0e21e
LW
667
668=item $ACCUMULATOR
669
670=item $^A
a054c801 671X<$^A> X<$ACCUMULATOR>
a0d0e21e
LW
672
673The current value of the write() accumulator for format() lines. A format
19799a22 674contains formline() calls that put their result into C<$^A>. After
a0d0e21e 675calling its format, write() prints out the contents of C<$^A> and empties.
14218588 676So you never really see the contents of C<$^A> unless you call
a0d0e21e
LW
677formline() yourself and then look at it. See L<perlform> and
678L<perlfunc/formline()>.
679
680=item $CHILD_ERROR
681
682=item $?
a054c801 683X<$?> X<$CHILD_ERROR>
a0d0e21e 684
54310121 685The status returned by the last pipe close, backtick (C<``>) command,
19799a22
GS
686successful call to wait() or waitpid(), or from the system()
687operator. This is just the 16-bit status word returned by the
e5218da5 688traditional Unix wait() system call (or else is made up to look like it). Thus, the
c47ff5f1 689exit value of the subprocess is really (C<<< $? >> 8 >>>), and
19799a22
GS
690C<$? & 127> gives which signal, if any, the process died from, and
691C<$? & 128> reports whether there was a core dump. (Mnemonic:
692similar to B<sh> and B<ksh>.)
a0d0e21e 693
7b8d334a 694Additionally, if the C<h_errno> variable is supported in C, its value
14218588 695is returned via $? if any C<gethost*()> function fails.
7b8d334a 696
19799a22 697If you have installed a signal handler for C<SIGCHLD>, the
aa689395
PP
698value of C<$?> will usually be wrong outside that handler.
699
a8f8344d
PP
700Inside an C<END> subroutine C<$?> contains the value that is going to be
701given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
19799a22
GS
702change the exit status of your program. For example:
703
704 END {
705 $? = 1 if $? == 255; # die would make it 255
706 }
a8f8344d 707
aa689395 708Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
ff0cee69 709actual VMS exit status, instead of the default emulation of POSIX
9bc98430 710status; see L<perlvms/$?> for details.
f86702cc 711
55602bd2
IZ
712Also see L<Error Indicators>.
713
e5218da5 714=item ${^CHILD_ERROR_NATIVE}
a054c801 715X<$^CHILD_ERROR_NATIVE>
e5218da5
GA
716
717The native status returned by the last pipe close, backtick (C<``>)
718command, successful call to wait() or waitpid(), or from the system()
719operator. On POSIX-like systems this value can be decoded with the
720WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED, WSTOPSIG
721and WIFCONTINUED functions provided by the L<POSIX> module.
722
723Under VMS this reflects the actual VMS exit status; i.e. it is the same
724as $? when the pragma C<use vmsish 'status'> is in effect.
725
0a378802 726=item ${^ENCODING}
a054c801 727X<$^ENCODING>
0a378802 728
740bd165
PN
729The I<object reference> to the Encode object that is used to convert
730the source code to Unicode. Thanks to this variable your perl script
731does not have to be written in UTF-8. Default is I<undef>. The direct
732manipulation of this variable is highly discouraged. See L<encoding>
048c20cb 733for more details.
0a378802 734
a0d0e21e
LW
735=item $OS_ERROR
736
737=item $ERRNO
738
739=item $!
a054c801 740X<$!> X<$ERRNO> X<$OS_ERROR>
a0d0e21e 741
19799a22 742If used numerically, yields the current value of the C C<errno>
6ab308ee
JH
743variable, or in other words, if a system or library call fails, it
744sets this variable. This means that the value of C<$!> is meaningful
745only I<immediately> after a B<failure>:
746
747 if (open(FH, $filename)) {
748 # Here $! is meaningless.
749 ...
750 } else {
751 # ONLY here is $! meaningful.
752 ...
753 # Already here $! might be meaningless.
754 }
755 # Since here we might have either success or failure,
756 # here $! is meaningless.
757
758In the above I<meaningless> stands for anything: zero, non-zero,
759C<undef>. A successful system or library call does B<not> set
760the variable to zero.
761
271df126 762If used as a string, yields the corresponding system error string.
19799a22
GS
763You can assign a number to C<$!> to set I<errno> if, for instance,
764you want C<"$!"> to return the string for error I<n>, or you want
765to set the exit value for the die() operator. (Mnemonic: What just
766went bang?)
a0d0e21e 767
55602bd2
IZ
768Also see L<Error Indicators>.
769
4c5cef9b 770=item %!
a054c801 771X<%!>
4c5cef9b
MJD
772
773Each element of C<%!> has a true value only if C<$!> is set to that
774value. For example, C<$!{ENOENT}> is true if and only if the current
3be065a1
JH
775value of C<$!> is C<ENOENT>; that is, if the most recent error was
776"No such file or directory" (or its moral equivalent: not all operating
777systems give that exact error, and certainly not all languages).
778To check if a particular key is meaningful on your system, use
779C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>.
780See L<Errno> for more information, and also see above for the
781validity of C<$!>.
4c5cef9b 782
5c055ba3
PP
783=item $EXTENDED_OS_ERROR
784
785=item $^E
a054c801 786X<$^E> X<$EXTENDED_OS_ERROR>
5c055ba3 787
22fae026
TM
788Error information specific to the current operating system. At
789the moment, this differs from C<$!> under only VMS, OS/2, and Win32
790(and for MacPerl). On all other platforms, C<$^E> is always just
791the same as C<$!>.
792
793Under VMS, C<$^E> provides the VMS status value from the last
794system error. This is more specific information about the last
795system error than that provided by C<$!>. This is particularly
d516a115 796important when C<$!> is set to B<EVMSERR>.
22fae026 797
1c1c7f20
GS
798Under OS/2, C<$^E> is set to the error code of the last call to
799OS/2 API either via CRT, or directly from perl.
22fae026
TM
800
801Under Win32, C<$^E> always returns the last error information
802reported by the Win32 call C<GetLastError()> which describes
803the last error from within the Win32 API. Most Win32-specific
19799a22 804code will report errors via C<$^E>. ANSI C and Unix-like calls
22fae026
TM
805set C<errno> and so most portable Perl code will report errors
806via C<$!>.
807
808Caveats mentioned in the description of C<$!> generally apply to
809C<$^E>, also. (Mnemonic: Extra error explanation.)
5c055ba3 810
55602bd2
IZ
811Also see L<Error Indicators>.
812
a0d0e21e
LW
813=item $EVAL_ERROR
814
815=item $@
a054c801 816X<$@> X<$EVAL_ERROR>
a0d0e21e 817
4a280ebe
JG
818The Perl syntax error message from the last eval() operator.
819If $@ is the null string, the last eval() parsed and executed
820correctly (although the operations you invoked may have failed in the
821normal fashion). (Mnemonic: Where was the syntax error "at"?)
a0d0e21e 822
19799a22 823Warning messages are not collected in this variable. You can,
a8f8344d 824however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
54310121 825as described below.
748a9306 826
55602bd2
IZ
827Also see L<Error Indicators>.
828
a0d0e21e
LW
829=item $PROCESS_ID
830
831=item $PID
832
833=item $$
a054c801 834X<$$> X<$PID> X<$PROCESS_ID>
a0d0e21e 835
19799a22
GS
836The process number of the Perl running this script. You should
837consider this variable read-only, although it will be altered
838across fork() calls. (Mnemonic: same as shells.)
a0d0e21e 839
4d76a344
RGS
840Note for Linux users: on Linux, the C functions C<getpid()> and
841C<getppid()> return different values from different threads. In order to
842be portable, this behavior is not reflected by C<$$>, whose value remains
843consistent across threads. If you want to call the underlying C<getpid()>,
e3256f86 844you may use the CPAN module C<Linux::Pid>.
4d76a344 845
a0d0e21e
LW
846=item $REAL_USER_ID
847
848=item $UID
849
850=item $<
a054c801 851X<< $< >> X<$UID> X<$REAL_USER_ID>
a0d0e21e 852
19799a22 853The real uid of this process. (Mnemonic: it's the uid you came I<from>,
a043a685 854if you're running setuid.) You can change both the real uid and
a537debe
SP
855the effective uid at the same time by using POSIX::setuid(). Since
856changes to $< require a system call, check $! after a change attempt to
857detect any possible errors.
a0d0e21e
LW
858
859=item $EFFECTIVE_USER_ID
860
861=item $EUID
862
863=item $>
a054c801 864X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
a0d0e21e
LW
865
866The effective uid of this process. Example:
867
868 $< = $>; # set real to effective uid
869 ($<,$>) = ($>,$<); # swap real and effective uid
870
a043a685 871You can change both the effective uid and the real uid at the same
a537debe
SP
872time by using POSIX::setuid(). Changes to $> require a check to $!
873to detect any possible errors after an attempted change.
a043a685 874
19799a22 875(Mnemonic: it's the uid you went I<to>, if you're running setuid.)
c47ff5f1 876C<< $< >> and C<< $> >> can be swapped only on machines
8cc95fdb 877supporting setreuid().
a0d0e21e
LW
878
879=item $REAL_GROUP_ID
880
881=item $GID
882
883=item $(
a054c801 884X<$(> X<$GID> X<$REAL_GROUP_ID>
a0d0e21e
LW
885
886The real gid of this process. If you are on a machine that supports
887membership in multiple groups simultaneously, gives a space separated
888list of groups you are in. The first number is the one returned by
889getgid(), and the subsequent ones by getgroups(), one of which may be
8cc95fdb
PP
890the same as the first number.
891
19799a22
GS
892However, a value assigned to C<$(> must be a single number used to
893set the real gid. So the value given by C<$(> should I<not> be assigned
894back to C<$(> without being forced numeric, such as by adding zero.
8cc95fdb 895
a043a685 896You can change both the real gid and the effective gid at the same
a537debe
SP
897time by using POSIX::setgid(). Changes to $( require a check to $!
898to detect any possible errors after an attempted change.
a043a685 899
19799a22
GS
900(Mnemonic: parentheses are used to I<group> things. The real gid is the
901group you I<left>, if you're running setgid.)
a0d0e21e
LW
902
903=item $EFFECTIVE_GROUP_ID
904
905=item $EGID
906
907=item $)
a054c801 908X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
a0d0e21e
LW
909
910The effective gid of this process. If you are on a machine that
911supports membership in multiple groups simultaneously, gives a space
912separated list of groups you are in. The first number is the one
913returned by getegid(), and the subsequent ones by getgroups(), one of
8cc95fdb
PP
914which may be the same as the first number.
915
19799a22 916Similarly, a value assigned to C<$)> must also be a space-separated
14218588 917list of numbers. The first number sets the effective gid, and
8cc95fdb
PP
918the rest (if any) are passed to setgroups(). To get the effect of an
919empty list for setgroups(), just repeat the new effective gid; that is,
920to force an effective gid of 5 and an effectively empty setgroups()
921list, say C< $) = "5 5" >.
922
a043a685
GW
923You can change both the effective gid and the real gid at the same
924time by using POSIX::setgid() (use only a single numeric argument).
a537debe
SP
925Changes to $) require a check to $! to detect any possible errors
926after an attempted change.
a043a685 927
19799a22
GS
928(Mnemonic: parentheses are used to I<group> things. The effective gid
929is the group that's I<right> for you, if you're running setgid.)
a0d0e21e 930
c47ff5f1 931C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
19799a22
GS
932machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
933and C<$)> can be swapped only on machines supporting setregid().
a0d0e21e
LW
934
935=item $PROGRAM_NAME
936
937=item $0
a054c801 938X<$0> X<$PROGRAM_NAME>
a0d0e21e 939
80bca1b4
JH
940Contains the name of the program being executed.
941
942On some (read: not all) operating systems assigning to C<$0> modifies
943the argument area that the C<ps> program sees. On some platforms you
944may have to use special C<ps> options or a different C<ps> to see the
945changes. Modifying the $0 is more useful as a way of indicating the
946current program state than it is for hiding the program you're
947running. (Mnemonic: same as B<sh> and B<ksh>.)
f9cbb277 948
cf525c36 949Note that there are platform specific limitations on the maximum
f9cbb277
JH
950length of C<$0>. In the most extreme case it may be limited to the
951space occupied by the original C<$0>.
a0d0e21e 952
80bca1b4
JH
953In some platforms there may be arbitrary amount of padding, for
954example space characters, after the modified name as shown by C<ps>.
dda345b7 955In some platforms this padding may extend all the way to the original
c80e2480
JH
956length of the argument area, no matter what you do (this is the case
957for example with Linux 2.2).
80bca1b4 958
4bc88a62 959Note for BSD users: setting C<$0> does not completely remove "perl"
6a4647a3
JH
960from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
961result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
962and the " (perl)" suffix are shown depends on your exact BSD variant
963and version). This is an operating system feature, Perl cannot help it.
4bc88a62 964
e2975953
JH
965In multithreaded scripts Perl coordinates the threads so that any
966thread may modify its copy of the C<$0> and the change becomes visible
cf525c36 967to ps(1) (assuming the operating system plays along). Note that
80bca1b4
JH
968the view of C<$0> the other threads have will not change since they
969have their own copies of it.
e2975953 970
a0d0e21e 971=item $[
a054c801 972X<$[>
a0d0e21e
LW
973
974The index of the first element in an array, and of the first character
19799a22
GS
975in a substring. Default is 0, but you could theoretically set it
976to 1 to make Perl behave more like B<awk> (or Fortran) when
977subscripting and when evaluating the index() and substr() functions.
978(Mnemonic: [ begins subscripts.)
a0d0e21e 979
19799a22
GS
980As of release 5 of Perl, assignment to C<$[> is treated as a compiler
981directive, and cannot influence the behavior of any other file.
f83ed198 982(That's why you can only assign compile-time constants to it.)
19799a22 983Its use is highly discouraged.
a0d0e21e 984
f83ed198 985Note that, unlike other compile-time directives (such as L<strict>),
af7a0647
RGS
986assignment to C<$[> can be seen from outer lexical scopes in the same file.
987However, you can use local() on it to strictly bind its value to a
f83ed198
RGS
988lexical block.
989
a0d0e21e 990=item $]
a054c801 991X<$]>
a0d0e21e 992
54310121
PP
993The version + patchlevel / 1000 of the Perl interpreter. This variable
994can be used to determine whether the Perl interpreter executing a
995script is in the right range of versions. (Mnemonic: Is this version
996of perl in the right bracket?) Example:
a0d0e21e
LW
997
998 warn "No checksumming!\n" if $] < 3.019;
999
54310121 1000See also the documentation of C<use VERSION> and C<require VERSION>
19799a22 1001for a convenient way to fail if the running Perl interpreter is too old.
a0d0e21e 1002
0c8d858b
MS
1003The floating point representation can sometimes lead to inaccurate
1004numeric comparisons. See C<$^V> for a more modern representation of
1005the Perl version that allows accurate string comparisons.
16070b82 1006
305aace0
NIS
1007=item $COMPILING
1008
1009=item $^C
a054c801 1010X<$^C> X<$COMPILING>
305aace0 1011
19799a22
GS
1012The current value of the flag associated with the B<-c> switch.
1013Mainly of use with B<-MO=...> to allow code to alter its behavior
1014when being compiled, such as for example to AUTOLOAD at compile
1015time rather than normal, deferred loading. See L<perlcc>. Setting
1016C<$^C = 1> is similar to calling C<B::minus_c>.
305aace0 1017
a0d0e21e
LW
1018=item $DEBUGGING
1019
1020=item $^D
a054c801 1021X<$^D> X<$DEBUGGING>
a0d0e21e
LW
1022
1023The current value of the debugging flags. (Mnemonic: value of B<-D>
b4ab917c
DM
1024switch.) May be read or set. Like its command-line equivalent, you can use
1025numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">.
a0d0e21e 1026
a3621e74
YO
1027=item ${^RE_DEBUG_FLAGS}
1028
1029The current value of the regex debugging flags. Set to 0 for no debug output
1030even when the re 'debug' module is loaded. See L<re> for details.
1031
0111c4fd 1032=item ${^RE_TRIE_MAXBUF}
a3621e74
YO
1033
1034Controls how certain regex optimisations are applied and how much memory they
1035utilize. This value by default is 65536 which corresponds to a 512kB temporary
1036cache. Set this to a higher value to trade memory for speed when matching
1037large alternations. Set it to a lower value if you want the optimisations to
1038be as conservative of memory as possible but still occur, and set it to a
1039negative value to prevent the optimisation and conserve the most memory.
1040Under normal situations this variable should be of no interest to you.
1041
a0d0e21e
LW
1042=item $SYSTEM_FD_MAX
1043
1044=item $^F
a054c801 1045X<$^F> X<$SYSTEM_FD_MAX>
a0d0e21e
LW
1046
1047The maximum system file descriptor, ordinarily 2. System file
1048descriptors are passed to exec()ed processes, while higher file
1049descriptors are not. Also, during an open(), system file descriptors are
1050preserved even if the open() fails. (Ordinary file descriptors are
19799a22 1051closed before the open() is attempted.) The close-on-exec
a0d0e21e 1052status of a file descriptor will be decided according to the value of
8d2a6795
GS
1053C<$^F> when the corresponding file, pipe, or socket was opened, not the
1054time of the exec().
a0d0e21e 1055
6e2995f4
PP
1056=item $^H
1057
0462a1ab
GS
1058WARNING: This variable is strictly for internal use only. Its availability,
1059behavior, and contents are subject to change without notice.
1060
1061This variable contains compile-time hints for the Perl interpreter. At the
1062end of compilation of a BLOCK the value of this variable is restored to the
1063value when the interpreter started to compile the BLOCK.
1064
1065When perl begins to parse any block construct that provides a lexical scope
1066(e.g., eval body, required file, subroutine body, loop body, or conditional
1067block), the existing value of $^H is saved, but its value is left unchanged.
1068When the compilation of the block is completed, it regains the saved value.
1069Between the points where its value is saved and restored, code that
1070executes within BEGIN blocks is free to change the value of $^H.
1071
1072This behavior provides the semantic of lexical scoping, and is used in,
1073for instance, the C<use strict> pragma.
1074
1075The contents should be an integer; different bits of it are used for
1076different pragmatic flags. Here's an example:
1077
1078 sub add_100 { $^H |= 0x100 }
1079
1080 sub foo {
1081 BEGIN { add_100() }
1082 bar->baz($boon);
1083 }
1084
1085Consider what happens during execution of the BEGIN block. At this point
1086the BEGIN block has already been compiled, but the body of foo() is still
1087being compiled. The new value of $^H will therefore be visible only while
1088the body of foo() is being compiled.
1089
1090Substitution of the above BEGIN block with:
1091
1092 BEGIN { require strict; strict->import('vars') }
1093
1094demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
1095version of the same lexical pragma:
1096
1097 BEGIN { require strict; strict->import('vars') if $condition }
1098
1099=item %^H
1100
0462a1ab 1101The %^H hash provides the same scoping semantic as $^H. This makes it
46e5f5f4 1102useful for implementation of lexically scoped pragmas. See L<perlpragma>.
6e2995f4 1103
a0d0e21e
LW
1104=item $INPLACE_EDIT
1105
1106=item $^I
a054c801 1107X<$^I> X<$INPLACE_EDIT>
a0d0e21e
LW
1108
1109The current value of the inplace-edit extension. Use C<undef> to disable
1110inplace editing. (Mnemonic: value of B<-i> switch.)
1111
fb73857a 1112=item $^M
a054c801 1113X<$^M>
fb73857a 1114
19799a22
GS
1115By default, running out of memory is an untrappable, fatal error.
1116However, if suitably built, Perl can use the contents of C<$^M>
1117as an emergency memory pool after die()ing. Suppose that your Perl
0acca065 1118were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
19799a22 1119Then
fb73857a 1120
19799a22 1121 $^M = 'a' x (1 << 16);
fb73857a 1122
51ee6500 1123would allocate a 64K buffer for use in an emergency. See the
19799a22 1124F<INSTALL> file in the Perl distribution for information on how to
0acca065
RGS
1125add custom C compilation flags when compiling perl. To discourage casual
1126use of this advanced feature, there is no L<English|English> long name for
1127this variable.
fb73857a 1128
5c055ba3 1129=item $OSNAME
6e2995f4 1130
5c055ba3 1131=item $^O
a054c801 1132X<$^O> X<$OSNAME>
5c055ba3
PP
1133
1134The name of the operating system under which this copy of Perl was
1135built, as determined during the configuration process. The value
19799a22
GS
1136is identical to C<$Config{'osname'}>. See also L<Config> and the
1137B<-V> command-line switch documented in L<perlrun>.
5c055ba3 1138
443f6d01 1139In Windows platforms, $^O is not very helpful: since it is always
7f510801
GS
1140C<MSWin32>, it doesn't tell the difference between
114195/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
1142Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
1143between the variants.
916d64a3 1144
e2e27056
JH
1145=item ${^OPEN}
1146
1147An internal variable used by PerlIO. A string in two parts, separated
fae2c0fb
RGS
1148by a C<\0> byte, the first part describes the input layers, the second
1149part describes the output layers.
e2e27056 1150
a0d0e21e
LW
1151=item $PERLDB
1152
1153=item $^P
a054c801 1154X<$^P> X<$PERLDB>
a0d0e21e 1155
19799a22
GS
1156The internal variable for debugging support. The meanings of the
1157various bits are subject to change, but currently indicate:
84902520
TB
1158
1159=over 6
1160
1161=item 0x01
1162
1163Debug subroutine enter/exit.
1164
1165=item 0x02
1166
1167Line-by-line debugging.
1168
1169=item 0x04
1170
1171Switch off optimizations.
1172
1173=item 0x08
1174
1175Preserve more data for future interactive inspections.
1176
1177=item 0x10
1178
1179Keep info about source lines on which a subroutine is defined.
1180
1181=item 0x20
1182
1183Start with single-step on.
1184
83ee9e09
GS
1185=item 0x40
1186
1187Use subroutine address instead of name when reporting.
1188
1189=item 0x80
1190
1191Report C<goto &subroutine> as well.
1192
1193=item 0x100
1194
1195Provide informative "file" names for evals based on the place they were compiled.
1196
1197=item 0x200
1198
1199Provide informative names to anonymous subroutines based on the place they
1200were compiled.
1201
7619c85e
RG
1202=item 0x400
1203
1204Debug assertion subroutines enter/exit.
1205
84902520
TB
1206=back
1207
19799a22
GS
1208Some bits may be relevant at compile-time only, some at
1209run-time only. This is a new mechanism and the details may change.
a0d0e21e 1210
66558a10
GS
1211=item $LAST_REGEXP_CODE_RESULT
1212
b9ac3b5b 1213=item $^R
a054c801 1214X<$^R> X<$LAST_REGEXP_CODE_RESULT>
b9ac3b5b 1215
19799a22
GS
1216The result of evaluation of the last successful C<(?{ code })>
1217regular expression assertion (see L<perlre>). May be written to.
b9ac3b5b 1218
66558a10
GS
1219=item $EXCEPTIONS_BEING_CAUGHT
1220
fb73857a 1221=item $^S
a054c801 1222X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
fb73857a 1223
fa05a9fd
IST
1224Current state of the interpreter.
1225
1226 $^S State
1227 --------- -------------------
1228 undef Parsing module/eval
1229 true (1) Executing an eval
1230 false (0) Otherwise
1231
1232The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.
fb73857a 1233
a0d0e21e
LW
1234=item $BASETIME
1235
1236=item $^T
a054c801 1237X<$^T> X<$BASETIME>
a0d0e21e 1238
19799a22 1239The time at which the program began running, in seconds since the
5f05dabc 1240epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
19799a22 1241and B<-C> filetests are based on this value.
a0d0e21e 1242
7c36658b
MS
1243=item ${^TAINT}
1244
9aa05f58
RGS
1245Reflects if taint mode is on or off. 1 for on (the program was run with
1246B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
18e8c5b0 1247B<-t> or B<-TU>). This variable is read-only.
7c36658b 1248
a05d7ebb
JH
1249=item ${^UNICODE}
1250
ab9e1bb7
JH
1251Reflects certain Unicode settings of Perl. See L<perlrun>
1252documentation for the C<-C> switch for more information about
1253the possible values. This variable is set during Perl startup
1254and is thereafter read-only.
fde18df1 1255
e07ea26a
NC
1256=item ${^UTF8CACHE}
1257
1258This variable controls the state of the internal UTF-8 offset caching code.
16d9fe92
NC
12591 for on (the default), 0 for off, -1 to debug the caching code by checking
1260all its results against linear scans, and panicking on any discrepancy.
e07ea26a 1261
ea8eae40
RGS
1262=item ${^UTF8LOCALE}
1263
1264This variable indicates whether an UTF-8 locale was detected by perl at
1265startup. This information is used by perl when it's in
1266adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
1267switch); see L<perlrun> for more info on this.
1268
44dcb63b 1269=item $PERL_VERSION
b459063d 1270
16070b82 1271=item $^V
a054c801 1272X<$^V> X<$PERL_VERSION>
16070b82
GS
1273
1274The revision, version, and subversion of the Perl interpreter, represented
da2094fd 1275as a string composed of characters with those ordinals. Thus in Perl v5.6.0
44dcb63b
GS
1276it equals C<chr(5) . chr(6) . chr(0)> and will return true for
1277C<$^V eq v5.6.0>. Note that the characters in this string value can
1278potentially be in Unicode range.
16070b82 1279
7d2b1222
DM
1280This variable first appeared in perl 5.6.0; earlier versions of perl will
1281see an undefined value.
1282
16070b82
GS
1283This can be used to determine whether the Perl interpreter executing a
1284script is in the right range of versions. (Mnemonic: use ^V for Version
44dcb63b 1285Control.) Example:
16070b82 1286
7d2b1222 1287 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
16070b82 1288
aa2f2a36
AMS
1289To convert C<$^V> into its string representation use sprintf()'s
1290C<"%vd"> conversion:
1291
1292 printf "version is v%vd\n", $^V; # Perl's version
1293
44dcb63b 1294See the documentation of C<use VERSION> and C<require VERSION>
16070b82
GS
1295for a convenient way to fail if the running Perl interpreter is too old.
1296
1297See also C<$]> for an older representation of the Perl version.
1298
a0d0e21e
LW
1299=item $WARNING
1300
1301=item $^W
a054c801 1302X<$^W> X<$WARNING>
a0d0e21e 1303
19799a22
GS
1304The current value of the warning switch, initially true if B<-w>
1305was used, false otherwise, but directly modifiable. (Mnemonic:
4438c4b7
JH
1306related to the B<-w> switch.) See also L<warnings>.
1307
6a818117 1308=item ${^WARNING_BITS}
4438c4b7
JH
1309
1310The current set of warning checks enabled by the C<use warnings> pragma.
1311See the documentation of C<warnings> for more details.
a0d0e21e 1312
2a8c8378
JD
1313=item ${^WIN32_SLOPPY_STAT}
1314
1315If this variable is set to a true value, then stat() on Windows will
1316not try to open the file. This means that the link count cannot be
1317determined and file attributes may be out of date if additional
1318hardlinks to the file exist. On the other hand, not opening the file
1319is considerably faster, especially for files on network drives.
1320
1321This variable could be set in the F<sitecustomize.pl> file to
1322configure the local Perl installation to use "sloppy" stat() by
1323default. See L<perlrun> for more information about site
1324customization.
1325
a0d0e21e
LW
1326=item $EXECUTABLE_NAME
1327
1328=item $^X
a054c801 1329X<$^X> X<$EXECUTABLE_NAME>
a0d0e21e 1330
e71940de 1331The name used to execute the current copy of Perl, from C's
21c1191d 1332C<argv[0]> or (where supported) F</proc/self/exe>.
38e4f4ae 1333
e71940de
PG
1334Depending on the host operating system, the value of $^X may be
1335a relative or absolute pathname of the perl program file, or may
1336be the string used to invoke perl but not the pathname of the
1337perl program file. Also, most operating systems permit invoking
1338programs that are not in the PATH environment variable, so there
a10d74f3
PG
1339is no guarantee that the value of $^X is in PATH. For VMS, the
1340value may or may not include a version number.
38e4f4ae 1341
e71940de
PG
1342You usually can use the value of $^X to re-invoke an independent
1343copy of the same perl that is currently running, e.g.,
1344
1345 @first_run = `$^X -le "print int rand 100 for 1..100"`;
1346
1347But recall that not all operating systems support forking or
1348capturing of the output of commands, so this complex statement
1349may not be portable.
38e4f4ae 1350
e71940de
PG
1351It is not safe to use the value of $^X as a path name of a file,
1352as some operating systems that have a mandatory suffix on
1353executable files do not require use of the suffix when invoking
1354a command. To convert the value of $^X to a path name, use the
1355following statements:
1356
304dea91 1357 # Build up a set of file names (not command names).
e71940de 1358 use Config;
68fb0eb7
PG
1359 $this_perl = $^X;
1360 if ($^O ne 'VMS')
1361 {$this_perl .= $Config{_exe}
1362 unless $this_perl =~ m/$Config{_exe}$/i;}
e71940de
PG
1363
1364Because many operating systems permit anyone with read access to
1365the Perl program file to make a copy of it, patch the copy, and
1366then execute the copy, the security-conscious Perl programmer
1367should take care to invoke the installed copy of perl, not the
1368copy referenced by $^X. The following statements accomplish
1369this goal, and produce a pathname that can be invoked as a
1370command or referenced as a file.
38e4f4ae
SB
1371
1372 use Config;
68fb0eb7
PG
1373 $secure_perl_path = $Config{perlpath};
1374 if ($^O ne 'VMS')
1375 {$secure_perl_path .= $Config{_exe}
1376 unless $secure_perl_path =~ m/$Config{_exe}$/i;}
a0d0e21e 1377
2d84a16a 1378=item ARGV
a054c801 1379X<ARGV>
2d84a16a
DM
1380
1381The special filehandle that iterates over command-line filenames in
1382C<@ARGV>. Usually written as the null filehandle in the angle operator
1383C<< <> >>. Note that currently C<ARGV> only has its magical effect
1384within the C<< <> >> operator; elsewhere it is just a plain filehandle
1385corresponding to the last file opened by C<< <> >>. In particular,
1386passing C<\*ARGV> as a parameter to a function that expects a filehandle
1387may not cause your function to automatically read the contents of all the
1388files in C<@ARGV>.
1389
a0d0e21e 1390=item $ARGV
a054c801 1391X<$ARGV>
a0d0e21e 1392
c47ff5f1 1393contains the name of the current file when reading from <>.
a0d0e21e
LW
1394
1395=item @ARGV
a054c801 1396X<@ARGV>
a0d0e21e 1397
19799a22 1398The array @ARGV contains the command-line arguments intended for
14218588 1399the script. C<$#ARGV> is generally the number of arguments minus
19799a22
GS
1400one, because C<$ARGV[0]> is the first argument, I<not> the program's
1401command name itself. See C<$0> for the command name.
a0d0e21e 1402
5ccee41e 1403=item ARGVOUT
a054c801 1404X<ARGVOUT>
5ccee41e
JA
1405
1406The special filehandle that points to the currently open output file
1407when doing edit-in-place processing with B<-i>. Useful when you have
1408to do a lot of inserting and don't want to keep modifying $_. See
1409L<perlrun> for the B<-i> switch.
1410
9b0e6e7a 1411=item @F
a054c801 1412X<@F>
9b0e6e7a
JP
1413
1414The array @F contains the fields of each line read in when autosplit
1415mode is turned on. See L<perlrun> for the B<-a> switch. This array
1416is package-specific, and must be declared or given a full package name
1417if not in package main when running under C<strict 'vars'>.
1418
a0d0e21e 1419=item @INC
a054c801 1420X<@INC>
a0d0e21e 1421
19799a22
GS
1422The array @INC contains the list of places that the C<do EXPR>,
1423C<require>, or C<use> constructs look for their library files. It
1424initially consists of the arguments to any B<-I> command-line
1425switches, followed by the default Perl library, probably
1426F</usr/local/lib/perl>, followed by ".", to represent the current
e48df184
RGS
1427directory. ("." will not be appended if taint checks are enabled, either by
1428C<-T> or by C<-t>.) If you need to modify this at runtime, you should use
19799a22
GS
1429the C<use lib> pragma to get the machine-dependent library properly
1430loaded also:
a0d0e21e 1431
cb1a09d0
AD
1432 use lib '/mypath/libdir/';
1433 use SomeMod;
303f2f76 1434
d54b56d5
RGS
1435You can also insert hooks into the file inclusion system by putting Perl
1436code directly into @INC. Those hooks may be subroutine references, array
1437references or blessed objects. See L<perlfunc/require> for details.
1438
314d39ce
MG
1439=item @ARG
1440
fb73857a 1441=item @_
a054c801 1442X<@_> X<@ARG>
fb73857a
PP
1443
1444Within a subroutine the array @_ contains the parameters passed to that
19799a22 1445subroutine. See L<perlsub>.
fb73857a 1446
a0d0e21e 1447=item %INC
a054c801 1448X<%INC>
a0d0e21e 1449
19799a22
GS
1450The hash %INC contains entries for each filename included via the
1451C<do>, C<require>, or C<use> operators. The key is the filename
1452you specified (with module names converted to pathnames), and the
14218588 1453value is the location of the file found. The C<require>
87275199 1454operator uses this hash to determine whether a particular file has
19799a22 1455already been included.
a0d0e21e 1456
89ccab8c
RGS
1457If the file was loaded via a hook (e.g. a subroutine reference, see
1458L<perlfunc/require> for a description of these hooks), this hook is
9ae8cd5b
RGS
1459by default inserted into %INC in place of a filename. Note, however,
1460that the hook may have set the %INC entry by itself to provide some more
1461specific info.
44f0be63 1462
b687b08b
TC
1463=item %ENV
1464
1465=item $ENV{expr}
a054c801 1466X<%ENV>
a0d0e21e
LW
1467
1468The hash %ENV contains your current environment. Setting a
19799a22
GS
1469value in C<ENV> changes the environment for any child processes
1470you subsequently fork() off.
a0d0e21e 1471
b687b08b
TC
1472=item %SIG
1473
1474=item $SIG{expr}
a054c801 1475X<%SIG>
a0d0e21e 1476
efbd929d 1477The hash C<%SIG> contains signal handlers for signals. For example:
a0d0e21e
LW
1478
1479 sub handler { # 1st argument is signal name
fb73857a 1480 my($sig) = @_;
a0d0e21e
LW
1481 print "Caught a SIG$sig--shutting down\n";
1482 close(LOG);
1483 exit(0);
1484 }
1485
fb73857a
PP
1486 $SIG{'INT'} = \&handler;
1487 $SIG{'QUIT'} = \&handler;
a0d0e21e 1488 ...
19799a22 1489 $SIG{'INT'} = 'DEFAULT'; # restore default action
a0d0e21e
LW
1490 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1491
f648820c
GS
1492Using a value of C<'IGNORE'> usually has the effect of ignoring the
1493signal, except for the C<CHLD> signal. See L<perlipc> for more about
1494this special case.
1495
19799a22 1496Here are some other examples:
a0d0e21e 1497
fb73857a 1498 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
a0d0e21e 1499 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
19799a22 1500 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
a0d0e21e
LW
1501 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1502
19799a22
GS
1503Be sure not to use a bareword as the name of a signal handler,
1504lest you inadvertently call it.
748a9306 1505
44a8e56a 1506If your system has the sigaction() function then signal handlers are
9ce5b4ad 1507installed using it. This means you get reliable signal handling.
44a8e56a 1508
9ce5b4ad
SG
1509The default delivery policy of signals changed in Perl 5.8.0 from
1510immediate (also known as "unsafe") to deferred, also known as
1511"safe signals". See L<perlipc> for more information.
45c0772f 1512
748a9306 1513Certain internal hooks can be also set using the %SIG hash. The
a8f8344d 1514routine indicated by C<$SIG{__WARN__}> is called when a warning message is
748a9306 1515about to be printed. The warning message is passed as the first
efbd929d
AT
1516argument. The presence of a C<__WARN__> hook causes the ordinary printing
1517of warnings to C<STDERR> to be suppressed. You can use this to save warnings
748a9306
LW
1518in a variable, or turn warnings into fatal errors, like this:
1519
1520 local $SIG{__WARN__} = sub { die $_[0] };
1521 eval $proggie;
1522
efbd929d
AT
1523As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
1524disable warnings using the empty subroutine:
1525
1526 local $SIG{__WARN__} = sub {};
1527
a8f8344d 1528The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
748a9306 1529is about to be thrown. The error message is passed as the first
efbd929d 1530argument. When a C<__DIE__> hook routine returns, the exception
748a9306 1531processing continues as it would have in the absence of the hook,
efbd929d 1532unless the hook routine itself exits via a C<goto>, a loop exit, or a C<die()>.
774d564b 1533The C<__DIE__> handler is explicitly disabled during the call, so that you
fb73857a
PP
1534can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1535
19799a22
GS
1536Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1537even inside an eval(). Do not use this to rewrite a pending exception
efbd929d 1538in C<$@>, or as a bizarre substitute for overriding C<CORE::GLOBAL::die()>.
19799a22
GS
1539This strange action at a distance may be fixed in a future release
1540so that C<$SIG{__DIE__}> is only called if your program is about
1541to exit, as was the original intent. Any other use is deprecated.
1542
1543C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1544they may be called to report (probable) errors found by the parser.
1545In such a case the parser may be in inconsistent state, so any
1546attempt to evaluate Perl code from such a handler will probably
1547result in a segfault. This means that warnings or errors that
1548result from parsing Perl should be used with extreme caution, like
1549this:
fb73857a
PP
1550
1551 require Carp if defined $^S;
1552 Carp::confess("Something wrong") if defined &Carp::confess;
1553 die "Something wrong, but could not load Carp to give backtrace...
1554 To see backtrace try starting Perl with -MCarp switch";
1555
1556Here the first line will load Carp I<unless> it is the parser who
1557called the handler. The second line will print backtrace and die if
1558Carp was available. The third line will be executed only if Carp was
1559not available.
1560
19799a22 1561See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
4438c4b7 1562L<warnings> for additional information.
68dc0745 1563
a0d0e21e 1564=back
55602bd2
IZ
1565
1566=head2 Error Indicators
a054c801 1567X<error> X<exception>
55602bd2 1568
19799a22
GS
1569The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1570about different types of error conditions that may appear during
1571execution of a Perl program. The variables are shown ordered by
1572the "distance" between the subsystem which reported the error and
1573the Perl process. They correspond to errors detected by the Perl
1574interpreter, C library, operating system, or an external program,
1575respectively.
55602bd2
IZ
1576
1577To illustrate the differences between these variables, consider the
19799a22 1578following Perl expression, which uses a single-quoted string:
55602bd2 1579
19799a22 1580 eval q{
22d0716c
SB
1581 open my $pipe, "/cdrom/install |" or die $!;
1582 my @res = <$pipe>;
1583 close $pipe or die "bad pipe: $?, $!";
19799a22 1584 };
55602bd2
IZ
1585
1586After execution of this statement all 4 variables may have been set.
1587
19799a22
GS
1588C<$@> is set if the string to be C<eval>-ed did not compile (this
1589may happen if C<open> or C<close> were imported with bad prototypes),
1590or if Perl code executed during evaluation die()d . In these cases
1591the value of $@ is the compile error, or the argument to C<die>
4cb1c523 1592(which will interpolate C<$!> and C<$?>). (See also L<Fatal>,
19799a22
GS
1593though.)
1594
c47ff5f1 1595When the eval() expression above is executed, open(), C<< <PIPE> >>,
19799a22
GS
1596and C<close> are translated to calls in the C run-time library and
1597thence to the operating system kernel. C<$!> is set to the C library's
1598C<errno> if one of these calls fails.
1599
1600Under a few operating systems, C<$^E> may contain a more verbose
1601error indicator, such as in this case, "CDROM tray not closed."
14218588 1602Systems that do not support extended error messages leave C<$^E>
19799a22
GS
1603the same as C<$!>.
1604
1605Finally, C<$?> may be set to non-0 value if the external program
1606F</cdrom/install> fails. The upper eight bits reflect specific
1607error conditions encountered by the program (the program's exit()
1608value). The lower eight bits reflect mode of failure, like signal
1609death and core dump information See wait(2) for details. In
1610contrast to C<$!> and C<$^E>, which are set only if error condition
1611is detected, the variable C<$?> is set on each C<wait> or pipe
1612C<close>, overwriting the old value. This is more like C<$@>, which
1613on every eval() is always set on failure and cleared on success.
2b92dfce 1614
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GS
1615For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1616and C<$?>.
2b92dfce
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1617
1618=head2 Technical Note on the Syntax of Variable Names
1619
19799a22
GS
1620Variable names in Perl can have several formats. Usually, they
1621must begin with a letter or underscore, in which case they can be
1622arbitrarily long (up to an internal limit of 251 characters) and
1623may contain letters, digits, underscores, or the special sequence
1624C<::> or C<'>. In this case, the part before the last C<::> or
1625C<'> is taken to be a I<package qualifier>; see L<perlmod>.
2b92dfce
GS
1626
1627Perl variable names may also be a sequence of digits or a single
1628punctuation or control character. These names are all reserved for
19799a22
GS
1629special uses by Perl; for example, the all-digits names are used
1630to hold data captured by backreferences after a regular expression
1631match. Perl has a special syntax for the single-control-character
1632names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1633character. For example, the notation C<$^W> (dollar-sign caret
1634C<W>) is the scalar variable whose name is the single character
1635control-C<W>. This is better than typing a literal control-C<W>
1636into your program.
2b92dfce 1637
87275199 1638Finally, new in Perl 5.6, Perl variable names may be alphanumeric
19799a22
GS
1639strings that begin with control characters (or better yet, a caret).
1640These variables must be written in the form C<${^Foo}>; the braces
1641are not optional. C<${^Foo}> denotes the scalar variable whose
1642name is a control-C<F> followed by two C<o>'s. These variables are
1643reserved for future special uses by Perl, except for the ones that
1644begin with C<^_> (control-underscore or caret-underscore). No
1645control-character name that begins with C<^_> will acquire a special
1646meaning in any future version of Perl; such names may therefore be
1647used safely in programs. C<$^_> itself, however, I<is> reserved.
1648
1fcb18de
RGS
1649Perl identifiers that begin with digits, control characters, or
1650punctuation characters are exempt from the effects of the C<package>
1651declaration and are always forced to be in package C<main>; they are
1652also exempt from C<strict 'vars'> errors. A few other names are also
1653exempt in these ways:
2b92dfce
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1654
1655 ENV STDIN
1656 INC STDOUT
1657 ARGV STDERR
5b88253b 1658 ARGVOUT _
2b92dfce
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1659 SIG
1660
1661In particular, the new special C<${^_XYZ}> variables are always taken
19799a22 1662to be in package C<main>, regardless of any C<package> declarations
747fafda 1663presently in scope.
2b92dfce 1664
19799a22
GS
1665=head1 BUGS
1666
1667Due to an unfortunate accident of Perl's implementation, C<use
1668English> imposes a considerable performance penalty on all regular
1669expression matches in a program, regardless of whether they occur
1670in the scope of C<use English>. For that reason, saying C<use
1671English> in libraries is strongly discouraged. See the
1672Devel::SawAmpersand module documentation from CPAN
1577cd80 1673( http://www.cpan.org/modules/by-module/Devel/ )
a054c801
GS
1674for more information. Writing C<use English '-no_match_vars';>
1675avoids the performance penalty.
2b92dfce 1676
19799a22
GS
1677Having to even think about the C<$^S> variable in your exception
1678handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1679invites grievous and difficult to track down errors. Avoid it
1680and use an C<END{}> or CORE::GLOBAL::die override instead.